CINCINNATI — The medical examiners are puzzled, and the bodies aren't giving up their secrets.

The bodies belong to overdose victims, and when they arrive on autopsy tables here, many are laced with substances that officials have never seen before. Drug screens weren't designed to identify them.

The drugs are synthetic opioids, a growing collection of chemicals cooked up in labs and responsible for an increasing number of overdose deaths across the country. In this city and elsewhere, they are spinning mysteries for the medical examiners who are called upon to identify them.

In some cases, bodies that by all indications shut down because of overdoses still show nothing on drug screens.

"We have to go back to the drawing board and say, 'What is this that's making this person overdose?'" said Dr. Thomas Gilson, the medical examiner in Cuyahoga County, which includes Cleveland.

The opioid epidemic has grabbed attention as a problem that demands responses from both public health and law enforcement officials. But these scientific puzzles, while being solved quietly in county morgues and city crime labs, are crucial if authorities are going to stem the crisis. If health officials know what drugs are ravaging communities, they can develop better prevention and treatment strategies. Identifying the drugs can also help law enforcement track the source.

One of the most common synthetic opioids is fentanyl, which is up to 100 times more potent than heroin. There are also so-called analogs of fentanyl, with slightly altered chemical structures that can be missed by drug screens.

Here in Ohio, a rash of overdoses has been attributed to an opioid known as carfentanil, which is 100 times more potent than standard fentanyl and which, until recently, was most commonly known as a sedative used by zoo veterinarians. It was blamed for a spike of 78 overdoses in Cincinnati over two days this summer.

Forensic pathologists say their job is complicated by the fact that they often have little information about how much of a drug was consumed or how soon before death. Even if there are witnesses, they may not have accurate information. In some cases, synthetic drugs are used to lace heroin or passed off as heroin to unsuspecting buyers.

"There are so many variables here and there's so much information that we don't get that it's really hard to evaluate scientifically what's significant and what isn't," said Dr. Lakshmi Sammarco, the coroner in Hamilton County, which includes Cincinnati.

As officials here tried to tease apart how many of the overdoses over the summer could be attributed to carfentanil, they were flying blind. Toxicologists needed a clean sample of the drug — what's called a standard — to adjust their screening procedures for it, and the companies that normally supply standards didn't have any. The lab wound up getting its first sample from the toxicology lab in Akron, which had gotten it from a zoo.

Eventually, the Drug Enforcement Administration also provided a standard.

AP Photo/Brynn Anderson

'No one has any idea'

Meanwhile, medical examiners had questions about how carfentanil affects the body and had to rely on limited scientific literature — just a few studies done in goats and antelopes.

"No one has any idea what level would cause someone to die," said Robert Topmiller, a Hamilton County toxicologist. "No one has any idea what level would cause somebody to be impaired. No one knows this stuff."

Since the summer, officials from Wisconsin to Florida have called Hamilton County asking how they adapted their screening protocol for carfentanil. But many labs still have to outsource confirmatory tests to private labs, officials say, creating another expense for these public agencies already crunched by a surge of opioid-related investigations.

Another problem with synthetic opioids is that lab equipment can prove insufficient to identify minuscule amounts of drugs in a victim's system, even if that amount is enough to prove deadly.

"You're actually pushing the limits of detection," said Gilson, the Cuyahoga County medical examiner.

Dr. James Gill, the chief medical examiner in Connecticut, said that people who overdose can go into a coma but keep breathing and pumping blood for hours. During that time, the drug keeps being metabolized, so at death, the level might actually be quite low. Plus, people can build up tolerance to drugs.

"There's a bit of a fallacy that there's this so-called fatal concentration," said Gill, who has studied the wide range of fentanyl concentrations overdose victims have shown.

Some officials say they are concerned about what's known as renarcotization.

The idea is that a drug like carfentanil lasts in people's systems so long that they can overdose, be revived with an antidote, and then hours later pass out without using drugs again.

Sammarco, the coroner in Hamilton County, said that she hasn't seen a definitive case yet, but has told local emergency crews to be on the lookout.

As she drove through Cincinnati on a recent day, heading back to her office from an event about the opioid epidemic's impact on children, Sammarco was talking about the presentations she gives to schools about drug use. She paused when she came across a police car and fire engine pulled to the side of the road with their lights flashing.